1. Field of the invention
The present invention relates to an antenna feeding circuit for an helical antenna, especially, a bifilar, quadrifilar and an octifilar helical antenna. A bifilar helical antenna is a helical antenna furnished with two filaments, a quadrifilar helical antenna is a helical antenna furnished with four filaments and an octifilar helical antenna is a helical antenna furnished with eight filaments.
2. Description of the prior art
Such an antenna feeding circuit is known, for example, from a "1/4 turn volute with split sheath balun" shown in FIG. 6 in "Resonant Quadrifilar Helix Antenna" disclosed in "Microwave Journal", December, 1970, p49-53.
FIG. 12 is a schematic perspective view of an antenna feeding circuit in the prior art, more specifically, it shows a 1/4 turn volute with split sheath balun disclosed in the article. In the figure, reference numerals 61, 62 denote, respectively, a first helical antenna element and a second helical antenna element. Reference numerals 63, 64 denote, respectively, a coaxial cable for feeding the helical antenna and a 1/4 wavelength slit disposed on the outer conductor of the coaxial cable 63. Reference numeral 65 denotes an impedance converter disposed on the inner conductor of the coaxial cable 63. Electric power is fed to the first and second helical antenna elements 61, 62 from an electric power feeding point 66.
Regarding the function, the first and second helical antenna elements 61, 62 can be assumed to be balanced lines, similar to a pair of parallel two lines. When unbalanced lines, for example, such as a coaxial cable, are connected to a helical antenna, a balance-unbalance converter is required between the helical antenna elements 61, 62 and the coaxial cable. Therefore, a balun, constituted by the coaxial cable 63, the 1/4 wavelength slit 64 and the impedance converter 65, is disposed as a balance-unbalance converter. Another function of this balun is to cancel out an inutile current, which appears when a balanced line is connected to an unbalanced line.
Japanese Patent Application 63-30006-A discloses another antenna feeding circuit, which comprises a 1/4 wavelength slit disposed on the outer conductor of a coaxial cable. The antenna comprises two sets of antenna elements, having an equal pitch angle, and each set of antenna elements is connected to one of two connecting portions of a connecting piece. The structure of this antenna feeding circuit facilitates the assembling of the antenna, and improves the preciseness of dimensions of the components of the antenna feeding circuit.
The antenna feeding circuits in the prior arts have following drawbacks due to such structures:
A balun is of a rather long dimension, i.e., 1/4 wavelength, in the longitudinal direction of an antenna;
The structure is rather complicated, as for example, when a coaxial structure of the antenna portion and the feeding circuit portion is employed to shorten the total length of the system including the length of the antenna portion.